1. Field of the invention
The present invention relates, in general, to a method for the production of biaxially stretched film and, more particularly, to an improvement in physical properties such as abrasion resistance, surface smoothness and slip property along with the method.
2. Description of the Prior Art
Polyester films can be widely utilized for magnetic applications such as video tapes, audio tapes, computer tapes and floppy discs, electrical applications such as condensers and electrical insulating pipes, package applications such as laminate films and metal vapor deposition films, and other applications such as microfilms, radio-photo films, architecture charts and stamping foils.
In magnetic applications, the polyester film is required to have a high level of surface smoothness and slip property. In last few years, the magnetic recording media have been rapidly improved, which demand the quality of base film. For example, in order to be applied to a magnetic tape such as a video tape, a polyester film, a base film, must have a highly smooth surface and a high data storage density.
For the improvement in travelling capability on processing, in slip property and in abrasion resistance, full well has it been known that outer particles, such as calcium carbonate, silica, alumina, zirconia, titan dioxide and inert polymer particles, are added.
Surface roughness of the polyester film which is obtained by the casting method must be kept in a suitable level. For example, if the surface roughness of a film is below a suitable level due to too small diameter of particle, a magnetic tape of the film is improved in electromagnetic transformation properties but worsened in travelling capacity. The inferiority of such property may lead to the generation of a great quantity of white powders and a great number of scratches. On the other hand, too large particles can enhance the travelling capacity of film but raises the surface roughness of film which results in the deterioration of electromagnetic transformation property such as drop-out.
To solve these problems, an effort has been made. For instance, at least two different particles which have different sizes were added. However, the particles are heterogeneous in shape and have a wide distribution of size, so that the surface roughness are non-uniform. In addition, huge protrusions on the film are apt to be formed, leading to the inferiority of the electromagnetic property of magnetic tape.